Antibiotics which specifically alter the synthesis of glycolipids and glycoproteins by either altering the synthesis of intermediates of protein glycosylation or changing the post-translational process were used to study the role of the carbohydrate portion of glycoproteins in virus synthesis and infectivity. It was found that the antibiotic streptovirudin which specifically inhibited glycosylation of viral glycoproteins also inhibited virion formation, however, the inhibitor swainsonine, which alters glycoproteins of vesicular stomatitis virus (VSV), had no effect either on virus formation or its infectivity. The effects of tumor promoters, such as the phorbol ester 12-0-tetradecanoyl-phorbol-13-acetate (TPA)-induced changes in cultured rat brain cells, were studied and the results suggested specific changes in ganglioside synthesis are associated with tumorgenicity.